Human natural killer (NK) cells and K cells mediating antibody-dependent cellular cytotoxicity have been shown to be large granular lymphocytes (LGL). The majority of LGL form lytic conjugates with a wide variety of NK-susceptible target cells. Interferon caused augmentation of NK and K cell activities of LGL and only LGL demonstrated either spontaneous or interferon-activated NK activity. Natural, recombinant and hybrid recombinant alpha, beta, and gamma interferon molecules have been shown to augment NK activity but vary widely in their potency relative to antiviral activity. A recombinant J species of IFN-Alpha has recently been shown to be unable to augment NK at a dose of 10,000 antiviral units; however, it was capable of augmentation of other leukocyte activities and demonstrated antiproliferative and antiviral activities similar to other IFN-Alpha's. This finding has led to studies regarding the structure-function relationship of IFN and NK boosting. IL-2, (T-cell growth factor), in addition to IFN, has demonstrated a potent ability to augment NK activity. This IL-2 mediated augmentation appears to be dependent on production of IFN-Gamma by LGL, since abrogation of antiviral activity with anti-IFN-Gamma serum abolishes NK boosting. Fresh in an attempt to examine this apparent heterogeneity, cultures and clones of highly purified LGL, grown in the presence of IL-2 have demonstrated morphology and cytotoxic patterns similar to fresh LGL. In addition to NK activity, cultured and clones of LGL have been shown to produce a variety of lymphokines (IL-1, IFN, CSF, BCGF).